Understanding How a Cam Works:

Legend:    

  TDC = Top Dead Center (Piston @ Highest point of travel in cylinder)
  BDC = Bottom Dead Center (Piston @ lowest point of travel in cylinder)
BTDC = Before top dead center
ATDC = After top dead center
BBDC = Before bottom dead center
ABDC = After bottom dead center 

     The camshaft which opens and closes the valves makes one complete revolution (360 degrees) while the crankshaft rotates TWICE (720 degrees). Camshaft timing is usually expressed in terms of crankshaft degrees relative to the piston location in the cylinder; Top Dead Center (TDC) & Bottom Dead Center (BDC).

 Valve Overlap

The number of degrees in cam shaft rotation that the intake & exhaust valve are open at the same time (The point in which the intake valve opening point BTDC and the Exhaust Cam's closing point ATDC).  Increasing the number of degrees of overlap tends to move the power band up the RPM band but you will sacrifice low end power. Increasing the overlap can increase peak power, but only if the exhaust system is efficient enough to properly scavenge the cylinder. Decreasing the overlap tends to boost lower rpm performance.

Lobe Centers

This is the angle between the intake and exhaust camshaft lobe peaks described in camshaft degrees.  This angle dictates two important events: the valve overlap around TDC, and intake or exhaust valve closure delay there is in the relevant stroke (inlet/exhaust). Tightening the lobe center angle produces more overlap around TDC and wider angles mean less overlap.

Valve Lift

Valve lift is the distance that the valve travels from the seat of the head to its farthest point. As the engine speed increases the need to increase valve lift is necessary to keep the inlet speeds from exceeding the Mach Index value of .6.  This is the point which volumetric efficiency falls off.
 

THE FIRST STROKE.

The piston starts from TDC and moves down the cylinder during the intake stroke while gaining velocity.  As it starts to reach the bottom of its stroke the speed then starts slowing down.  As the piston is moving down the cylinder the intake valve is opening. The air & gas mixture starts flowing into the cylinder as the intake valve opens, but the largest volume enters when the pressure differential is the greatest. This occurs when maximum piston velocity is reached which is somewhere between 70 to 80 degrees ATDC. What governs piston velocity is the stroke, rod length, RPM, and piston pin off-set. The maximum piston speed of the engine is then limited by the resistance to gas flow of the engine and/or the stresses due to the inertia of the moving parts.

FACT ONE: Volumetric efficiency is directly related to piston velocity!

Volumetric efficiency is a measure of the effectiveness of an engine's intake system.  The intake valve is almost closed as the piston reaches BDC, but it does not close completely until after BDC, when the piston is on its way back up the cylinder. The reason for this is because the incoming air/fuel mixture still has momentum even though the piston has slowed way down. We are now starting

THE SECOND STROKE.

The piston compresses the air/fuel mixture to a high enough pressure and temperature to allow ignition by means of the spark plug ignition.  By introducing the spark from the plug causes an explosion which produces the power & starts the downward travel of the piston which starts the

THE THIRD STROKE.

Power is produced while the gases in the cylinder expand and cool. In most cases, the gases are at a relatively low pressure by the time the crankshaft reaches 90 degrees After Top Dead Center (ATDC), so we can safely open the exhaust valve Before Bottom Dead Center (BBDC) to take advantage of blow- down. Otherwise, the piston would have to push all the exhaust out. When the piston reaches BDC we begin

THE FOURTH STROKE.

The exhaust valve is opening at a fairly rapid rate, the piston is going up and if the exhaust valve is not open a lot by the time the piston reaches maximum velocity there will be resistance in the cylinder caused by excessive exhaust gas pressure. This produces conditions which are referred to as pumping losses. As the piston reaches the top of the cylinder or the end of the fourth stroke, you will see the exhaust valve is almost closed but the intake valve is just beginning to rise off the seat! At TDC or the end of the fourth stroke both the intake and exhaust valves are open just a little, this is referred to as valve OVERLAP.